In LED devices, a LED chip is encapsulated in an encapsulant for providing protection from moisture and corrosive gases in the air. Currently, epoxy resins and silicone resins are employed as encapsulant (cf., Patent Documents 1-4, for example).
However, epoxy resins are poorly resistant to light and heat, and prone to change in color, thus posing problems, especially when employed in white LEDs which create white light based on near-ultraviolet to blue LED chips, that is, degradation and change in color over time due to light having the shorter wavelengths and thereby lowering intensity of light achieved. Degradation of epoxy resins is caused mainly by degeneration of the resins' own organic functional groups, and it is thought that the degradation is accelerated by catalysts and curing accelerators contained in the resins, as well as by unreacted functional groups remaining in the resins. Therefore, attempts have been made to change the types of the monomers employed in forming epoxy resins from those having aromatic rings, which are said to be prone to cause color changes, to those having aliphatic rings, which are free of such risks. However, no epoxy resins with a satisfying performance have been obtained so far.
Silicone resins, on the other hand, are superior in heat resistance and light resistance. However, they have high permeability to water vapor, and thus are incapable of guarding LED chips and phosphors from moisture, and they also have problems that they, owing to their poor adhesiveness, entail risks of their becoming detached from chip surfaces.
Furthermore, refractive indexes of resins are low as compared with those of semiconductors which form LED chips, and there is a great difference in refractive index between them. For example, where a GaN-based LED chip is encapsulated in a silicone resin, the difference in refractive index between GaN (2.3 to 2.4) and a silicone resin (refractive index: 1.4) is as much as about 0.9 to 1.0. The greater is the difference in refractive index between a LED chip and its encapsulant, the smaller the critical angle of total internal reflection becomes at the interface between the LED chip and the encapsulant. This results in more of the incident light arriving at the interface from the LED chip side at angles greater than the critical angle and thus being totally reflected, which leads to lowered output efficiency of light due to its repeated reflection within the LED chip accompanied by increased absorption within the chip. Moreover, such an increased absorption of light causes a problem of increased heating of the LED chip, which leads to unnecessary rise of the chip's temperature.
A white LED device is generally constructed in a way in which particles of multiple inorganic phosphors are dispersed in the encapsulant, which also serves as a pathway to transmit the light emitted from the LED chip, and a white light source is to realized as a whole by mixing colors of fluorescent light emitted from the particles excited by the light from the LED chip, or by further mixing it with the original light from the LED chip. A number of such inorganic phosphors are known based on, e.g., oxide-based, nitride-based, oxide/nitride-based, sulfide-based phosphors or so on. Among them, sulfide-based phosphors, such as (Ca, Sr)S:Eu, CaGa2S4:Eu, ZnS:Cu, Al, and the like, have high potential capacity of being utilized as phosphors to provide superior white light sources, for many of them efficiently emit vivid light when excited by blue light. However, phosphors based on sulfide and silicate, in particular sulfide-based phosphors, are hygroscopic and have a drawback that they easily degenerate by moisture. The resins mentioned above have only insufficient moisture resistance to maintain the stability of sulfide- or silicate-based phosphors added to them, and silicone resins are particularly inadequate because of their especially high permeability to water vapor. Therefore, resin-based encapsulants have the shortcoming that sulfide- and silicate-based phosphors cannot effectively utilized with them.
On the other hand, in order to avoid the problem of changing in color of encapsulant, it is known to employ, instead of epoxy resins, transparent low melting point glass as an encapsulant, whose melting point is adjusted to 130 to 350° C. by addition of selenium, thallium, arsenic, sulfur and the like (Patent Document 5). It is also known to encapsulate a LED chip in lead glass as a low melting point glass, whose melting point is at about 400° C. (Patent Document 6). It is also known to encapsulate a substrate on which a LED chip is mounted in a low melting point flat glass layer based on B2O3—SiO2—ZnO—Bi2O3—La2O3 or B2O3—SiO2—ZnO—Bi2O3—Nb2O3, by hot pressing (Patent Document 7). In this case, the deformation point of the low melting point glass is in the range of 475-534° C. according to the working examples presented there.
However, in order to melt the aforementioned low melting point glass, though they are claimed to have low melting points, must be heated at least to 130-350° C. or around 400° C. (Patent Document 5 or 6), or at least 475-534° C., which poses an unnegligible risk of thermal damages to the chips and the underlying substrates.
Thus, there has been a potential need for a transparent encapsulant which is stable against light and heat, highly moisture resistant and thereby substantially blocks moisture permeation, and yet can provide encapsulation even by heating at lower temperatures than conventional low melting point glass. Further, when used in direct contact with the surface of the semiconductor of which a LED chip is formed, an encapsulant having the higher refractive index is desired so as to reduce the difference between it and the refractive index of the semiconductor forming the chip.